1. Field of the Invention
The present invention relates to a high velocity burner, and more particularly, a variable exit high velocity burner. The variable exit provides a mechanism for controlling the actual flame exit velocity over a wide range of burner operation variables.
2. Description of Related Art
High velocity burners are a heating source preferred for most industrial furnaces and kilns. In these types of burners, such as those described in U.S. Pat. Nos. 5,570,679 and 5,263,849, fuel and combustion air are mixed with and ignited in a high-heat-resistant combustion chamber. The resultant hot combustion gases flow at high velocity through a nozzle-shaped outlet into the heating chamber, which is formed either by the furnace chamber itself or by some other device. The outlet of the combustion chamber can be constricted in nozzle-like fashion or formed by a nozzle ring. The mechanical energy of the gas stream or jet emerging from the combustion chamber, which derives predominantly from the fuel and nozzle shape, serves to mix and circulate the gases in the heating chamber, which in turn promotes the temperature equalization in a desired manner. Exit flame velocity for this type of burner is in excess of 25,000 ft/min when then burner is at high fire. The high velocity promotes good flue gas circulation in the furnace or kiln resulting in good temperature uniformity.
In high velocity burners, however, the burners cannot always be operated at their maximum firing rate. The flame exit velocity decreases as the burner firing rate is decreased. Actual flame exit velocity is a function of flow rate, pressure drop, combustor exit area and flame temperature. Accordingly, it would be advantageous to control the flame exit velocity in a high velocity burner to account for these variables.
There are several types of known flow control devices for conventional burners. For example, U.S. Pat. No. 2,565,039 shows a burner having a nozzle member as a part of a mixing valve. A needle having a tapered end cooperates with an aperture to increase or decrease the area between the outer size of the tapered end of the needle. The area of the opening of the burner is adjusted by rotating a head on the threaded stud to obtain the desired opening for best performance.
U.S. Pat. No. 3,663,153 shows an LTG burner in which a central rod is provided with a cone-shaped downstream end to vary the width of the primary air supply opening. A similar arrangement is shown in the acid gas burner of U.S. Pat. No. 3,782,884.
Other movable plug arrangements are seen in U.S. Pat. No. 4,902,222 in which a conical valve body cooperates with a valve seat. The valve rod is moved in a predetermined manner to decrease air flowing through the bypass conduit and allowing more air to flow into the mixing cup and the combustion chamber. U.S. Pat. No. 5,292,244 also shows that the use of Venturi mixer formed by a movable hollow body to vary the fuel/air mixture introduced into a combustion chamber is well known. However, these devices are used primarily to control the flow of combustible material in or around the combustion chamber, and not at the flame exit area of the burner.
What is desired is an adjustable velocity exit area for a high velocity burner to adjust the flame exit velocity or to maintain a constant velocity over a range of modulated firing rates. Such an adjustment would allow the flame exit velocity to be optimized over a wide range of operating conditions once the burner is installed in a furnace or kiln.